Carbon Cycling in Mangrove Restorations of Veracruz and Southeastern Florida

Abstract

Mangroves are an excellent naturally-occurring carbon sink due to their ability to accrete sediment while making major biomass and nutrient investments both aboveand below-ground, with rates of carbon burial exceeding those of other marine vegetation and terrestrial forests. Retention and restoration of mangrove forests offer the potential for perhaps the highest rates of atmospheric carbon reduction and carbon sequestration per area. However, mangroves and other coastal vegetative ecosystems are disappearing at an alarming rate due to habitat conversion and pollution, and may be lost entirely within the next 100 years if efforts are not made to reduce trends in mangrove degradation. Because restoration increases the area of mangrove cover sequestering carbon, it has the most potential for offsetting carbon emissions and mitigating the effects of climate change. To enact efficient and sound policy regarding decisions of converting, conserving or rehabilitating mangrove cover, it is important that mangrove carbon stocks and cycling are quantified worldwide. This study estimated carbon stocks in natural and restored mangrove soils in two regions around the Gulf of Mexico, in Veracruz, Mex. and South Florida, U.S.A., through 20-cm sediment core extraction and dry sediment carbon elemental analysis. Because the study includes natural and restored mangroves from the same locality, as well as areas still barren from the pollution events that necessitated restoration, it provides the unique opportunity to observe the complete progression of carbon stocks from natural to polluted to restored. I found that carbon losses due to pollution varied significantly (P < 0.05) by pollution type and in relation to the original standing natural mangrove carbon stocks. On average, sites in Florida lost 9.44 Mg C ha with pollution, and Veracruz sites lost 40.29 Mg C ha. I also found that restoration had the potential for recapturing C on average, and that active mangrove planting had the potential to restore even more carbon in upper sediment layers than pre-existing adjacent natural stocks, gaining 21.33 Mg C ha. These sites showed trends of active mangrove seedling planting resulting in faster carbon burial rates than passive recolonization, suggesting that the ability for mangroves to recolonize on their own could serve as an indicator of ideal sites for more active forms of mangrove restoration. INTRODUCTION 1 BACKGROUND (BLUE CARBON) 1 MANGROVES 3 RANGE AND PHYSIOLOGY 3 HABITAT CONVERSION 5 FOREST PROTECTION AND REHABILITATION 6 PROJECT DESIGN 8 LOCATIONS 10 FLORIDA 12 VERACRUZ 15 METHODS 18 FIELD SAMPLING 18 LAB ANALYSIS 20 RESULTS 26 SEDIMENT COMPOSITION 28 % ORGANIC CARBON 28 MOLAR C/N RATIOS 30 % CACO3 31 SEDIMENT PROPERTIES 32 BULK DENSITY 32 % LOI 33 DEPTH PROFILES 34 ORGANIC CARBON STOCKS 37 DISCUSSION 39 REGIONAL CONTROLS ON SOIL C STORAGE 39 POLLUTION TYPE INFLUENCES CARBON LOSSES 42 SITE AND TYPE OF RESTORATION INFLUENCE CARBON GAINS 45 C/N AND CACO3 AS BIOLOGICAL INDICATORS 50 CLIMATE MITIGATION AND CARBON PAYMENT 53